1. Field of the Invention
The present invention relates generally to time-base corrector circuits for time-base correction of a video signal, and more particularly to a time-base corrector circuit having the function of storing a video signal and outputting a still picture based on the signal.
2. Description of the Background Art
Appliances such as VTR generally record/reproduce information by means of high-speed contacting between the reproducing head and a recording tape, and therefore, the time-base of a reproduced signal may vary. This phenomenon is called jitter.
Meanwhile, a color component of an NTSC television signal is, for example, formed of orthogonally modulated chrominance subcarriers, and the phases of chrominance subcarriers convey information. More specifically, the phases correspond to hues. If, therefore, the phases of chrominance subcarriers are shifted by a jitter, correct hues cannot be reproduced.
A circuit for removing such a jitter from a reproduced signal is a so-called time-base corrector circuit (hereinafter referred to as TBC circuit).
FIG. 17 is a schematic block diagram showing the structure of a conventional TBC circuit.
The TBC circuit should generally perform signal processings for luminance signals and color signals. However, since the present invention is directed to color-related signal processings, the components for luminance-related signal processings are omitted, and only the color-related signal processings are detailed.
"Color carrier signal" in the following is produced by subjecting two chrominance subcarriers having the same frequency and a phase difference of 90.degree. from each other to orthogonal-binary-phase-modulation with separate color difference signals R-Y and B-Y and combining them.
A TBC circuit 600 includes a color modulator 1 for outputting color difference signals R-Y and B-Y based on a received video signals input. Herein, signal R represents a signal corresponding to color red, signal B represents a signal corresponding to blue, signal Y represents a luminance signal, signal R-Y a first color difference signal, and B-Y a second color difference signal.
TBC circuit 600 further includes an A/D converter 2 converting a received signal R-Y into a digital signal, an A/D converter 3 converting a received signal B-Y into a digital signal, a synchronizing signal separation circuit separating a horizontal synchronizing signal HW and a vertical synchronizing signal VW in a video signal, and a memory controller 8 for controlling the operations of A/D converters 2 and 3 in synchronization with these separated horizontal synchronizing signal HW and vertical synchronizing signal VW.
TBC circuit 600 further includes a memory circuit 4 controlled by memory controller 8 for receiving and storing the digital signals output from A/D converters 2 and 3, a reference signal generator 11 outputting to memory controller 8 a horizontal synchronizing reference signal HR and a vertical synchronizing reference signal VR corresponding to a read clock and generating a chrominance subcarrier SC, D/A converters 5 and 6 controlled by memory controller 8 for converting the digital signals from memory circuit 4 into analog signals, R-Y signal and B-Y signal.
TBC circuit 600 further includes an encoder 7 for adding color burst signals to signals R-Y and B-Y output from D/A converters 5 and 6, respectively, based on a burst flag pulse BFP output from reference signal generator 11.
Herein, the color burst signal functions as a phase reference for a color carrier signal conveying a color difference signal, upon reproducing a color signal in a video signal.
FIG. 18 is a waveform chart showing a horizontal blanking period being enlarged within a video signal waveform according to NTSC standards. The portion before the horizontal synchronizing signal is called front porch, and the portion after is called back porch. Since the color television system has been developed following the monochrome television system, a color burst signal to be a phase reference for a carrier color signal is overlayed taking advantage of the period of the back porch, in order to maintain compatibility with a monochrome video signal.
In the conventional TBC circuit 600, a color difference signal is A/D-converted in synchronization with a clock signal having a jitter the same as a reproduced video signal and then written in memory circuit 4. More specifically, a color difference signal corresponding to each pixel is written at an address in the memory corresponding to each position in the frame, and therefore most of the jitter will have been removed as the color difference signal is written in memory circuit 4.
As described above, the color difference signal written in memory circuit 4 is read out from memory circuit 4 in synchronization with signals HR and VR having stable phases, digital-analog converted and output, and therefore a resultant video signal will not be affected by the jitter even if the time base of the video signal is shifted at the time of reproduction.
Among such TBC circuits for time-base correction of a video signal, some perform signal processings after demodulating color signals in a video signal into color difference signals as described above, and others directly processes a composite video signal including a synchronizing signal, a color carrier signal and a color burst signal as disclosed by Japanese Patent Laying-Open No. 63-171094.
Some of the former TBC circuits have the function of taking in a still picture for output. Such a TBC circuit is generally designed such that if a monochrome video signal without a color burst signal is input as an input video signal, an output video signal is always added with a burst signal.
One of the reasons for always adding a color burst signal to an output video signal is as follows. Color demodulator 1 in a preceding stage to the memory demodulates a color signal and outputs a color difference signal with horizontal blanking. More specifically, only the signal portion corresponding to the scanning period among the video signal is output, and the signal corresponding to the horizontal blanking period is not stored in memory circuit 4. As a result, information about the presence/absence of a color burst signal is dropped from the signal input to memory circuit 4.
Therefore, in a succeeding stage to memory circuit 4, a color burst signal is added irrespectively of whether or not a color burst signal is present in an input video signal when the video signal is encoded.
Such a structure, however, adds a color burst signal to a monochrome video signal, and minute rainbow-like noises appear in a picture output at the TV receiver, which may degrade the resolution.
Meanwhile, video signals for a VTR, for example, are edited while dubbing the information into separate tapes for every cut, in other words, a so-called electronic editing is generally performed. In such electronic editing, scenes cut out from the initially cut out original tape on which the program is based are arranged in an appropriate order, in other words, an assemble editing is carried out. In such a case, a black burst tape which prerecords a black burst signal, i.e., a black level video signal having a synchronizing signal and a burst signal should be produced before editing. The scenes cut out from the original tape are dubbed to the black burst tape further recorded with time codes.
According to a conventional method, the black burst tape is produced by recording a tape with a black burst signal generated using a video signal generation circuit having an oscillator.
Meanwhile, some TBC circuits for time-base correction of a video signal have the function of outputting a video image taken into the memory in the form of a still picture. Such a TBC circuit includes reference signal generator 11, by which a color burst signal is generated and added to a video signal. Japanese Patent Laying-Open No. 62-287780 discloses a TBC circuit which outputs a black burst signal by switching an output signal to a blanking signal when a video signal is not input.
However, if a black burst tape for editing is produced using a conventional VTR device, such a black burst signal cannot be generated in a desired timing based on the idea of the editor.